Combustion boilers use solid fuels, such as coal, bark, biomass trimmings, wood or other biomass pellets, sawdust, tire derived fuel, refuse, straw, bagasse, or combinations of these, sometimes accompanied by fossil fuels. In many cases, these fuels either have high initial moisture content, or are stored outdoors exposed to rain and snow. In these cases, the fuels may contain water (or even ice) content which is too high for proper burning in a combustion boiler as commonly used by industry and utilities for generation of steam to perform chemical processes and/or to generate electricity.
To reduce the moisture content of dry fuels prior to their introduction into the combustion chambers of boilers, various types of fuel dryers are commonly employed. Most fuels dryers can be classified as a direct dryer or an indirect dryer. Direct dryers heat and dry the fuel by direct contact with the heat-providing fluid, which may be steam and/or hot air. Indirect dryers separate the wet fuel from the heat source using a heat exchange surface.
The choice of type of dryer depends on the biomass characteristics and the economics of the particular application of the boiler being supplied by the fuel. The advantages of drier fuel include higher efficiency, lower air emissions and improved boiler operation. Various types of dryers are employed, the main types being rotary dryers, flash dryers, and superheated steam dryers. Each dryer type has advantages depending on the material size, allowable space for the dryer, energy usage, fire risk minimization, environmental considerations (air emissions and generation of wastewater), the possibility of integrating the dryer to the process, and finally added costs.
The principle benefit of burning drier fuels is to increase the thermal efficiency of the boiler, thereby enabling reduced fuel consumption for the amount of steam produced. This increase in efficiency occurs through the higher flame temperatures possible when burning drier fuels. This benefit arises since with wet fuel some of the combustion heat is necessarily used to evaporate the water (and possibly melt the ice) out of the fuel prior to burning. Higher flame temperatures have multiple benefits, including larger thermal gradients for radiant heat transfer (which goes as the fourth-power of temperature, where the temperature is measured from absolute zero)—thus for the same amount of heat transfer, smaller banks of steam-generating tubes may be employed. Higher flame temperatures enhance combustion, producing lower carbon-monoxide levels and reduced fly ash leaving the boiler. Also, a higher percentage of the total energy content of the fuel is released at higher combustion temperatures—this may enable the usage of smaller fire boxes and lower-capacity ash handling systems. Further benefits of higher combustion temperatures include less need for excess combustion air while still maintaining acceptable exhaust opacity and CO levels. Less need for combustion air may enable use of smaller forced draft or induced draft blowers.
However, there are some valid concerns with using dried fuel. The higher combustion temperatures afforded by the use of pre-dried fuel may lead to slag formation (fusion of ash). In the prior art, problems with the dryer (causing the fuel to be inadequately dried, or not dried at all) had the potential to lead to wetter fuels being introduced to the boiler than it was designed for. Higher combustion temperatures may also accelerate corrosion through the formation of sulfuric acid.